The field of the invention relates to optical channel equalizers such as those used in optical networks.
First-generation optical networks employed a single-channel, in which an optical signal was carried by a single wavelength of light. The next-generation optical networks employed two channels, in which two optical signals were carried by two different wavelengths of light. Presently, optical networks employ multiple or N channels, in which N optical signals are carried by N different wavelength of light.
The N different wavelengths of light for the N channels are typically generated by N different laser sources (e.g., distributed feedback or xe2x80x9cDFBxe2x80x9d lasers) at different physical locations in the network. Each channel is individually modulated to create the optical signal for the channel. The N different channels are then multiplexed with an N-channel wavelength division multiplexer to send the channels over a single optical fiber, e.g., for long-haul optical communications.
The nature of N-channel optical networks is such that the power carried by the different channels naturally becomes increasingly non-uniform. There are three main factors that contribute to channel power non-uniformity. The first is light source non-uniformity, wherein the N different light sources associated with the N different channels have slightly different power outputs even if they are otherwise identical. Variable optical attenuators (VOAs) are typically used to uniformize the output powers of the different light sources.
The second factor contributing to channel power non-uniformity is called xe2x80x9cwavelength-dependent attenuation,xe2x80x9d wherein each channel (i.e., wavelength) is attenuated to a different degree when traveling over the optical network.
The third factor is called xe2x80x9cwavelength-dependent amplification,xe2x80x9d wherein each channel is amplified to a different degree when the signals pass through the optical amplifiers in the network.
In an N channel optical network, the channel power non-uniformity must be controlled to within a certain tolerance (e.g., within a few percent), or the network performance will suffer. This is because at one or more points in the optical network, the N channels are demultiplexed and fed to N different detectors. Each detector is designed to detect optical power between a maximum power PMAX and a minimum power PMIN. If the power in the channel exceeds PMAX, the detector saturates and cannot detect the bits in the optical signal. Likewise, if the power in the channel is below PMIN, the detector is not sensitive enough to detect the bits in the optical signal. The failure to detect bits present in an optical signal leads to an increase in the bit-error rate (BER) and thus diminished network performance.
Because the channel power must be within a select tolerance, the channel power non-uniformities must be compensated, i.e., the channel power must be xe2x80x9cequalized.xe2x80x9d To this end, N channel optical networks typically include one or more channel equalizers in the form of dynamic gain equalization filters (DGEFs) that equalize the channel powers. Each DGEF includes optical taps and detectors that tap and detect a small amount (e.g., 1%) of power from each channel of a demultiplexed signal to measure the power in each channel. Each channel is then passed through a corresponding VOA, which selectively attenuates the channel power in proportion to the measured power. The power-equalized channels are then re-multiplexed, and they continue their journey over the network.
While effective, this dynamic approach to channel equalization is complex and expensive because it requires VOAs and DGEFs. Future N channel optical networks would benefit from less complex and less expensive channel equalization devices and methods.